Research On Difference In Mode Of In-Line MZI And Its Application To Vernier Sensitization Sensing

Temperature and refractive index are important parameters in industrial production and biochemical research.In-Line Mach-Zehnder interferometer（In-Line MZI）has attracted much attention as it integrates the sensing and reference arms of a traditional MZI into a single fiber.The output signal of an In-Line MZI is a superimposed of different modal interference,and the spectral response to environment is a comprehensive behavior of different modes.Usually sensing test based on an In-Line MZI directly ascribes the wavelength shift or intensity variation of a characteristic dip in its spectrum to the change of measured,with the discrepancy between different modes and modal interference ignored.In this dissertation,discrepancy of modes and modal interference are studied.Based on the discrepancy,an In-Line MZI with enhanced refractive index sensitivity and reduced temperature cross talk,composed by two cascaded SMF-SMF-SMF unit interformeters,is developed for refractive index sensing,and another In-Line MZI with enhanced temperature sensitivity,cascaded by a SMF-SMF-SMF and a SMF-FMF-SMF,is designed for temperature sensing.The content is summarized as follows:（1）The core and cladding mode field of a single mode fiber（SMF）and a few mode fiber（FMF）are simulated and analyzed.The values of effective refractive index of different modes under different ambient refractive index,temperature and incident wavelength are calculated.Based on the calculation,the effective refractive index difference between fundamental-cladding mode and cladding-cladding mode in the SMF,and LP₀₁-LP₁₁ mode in the FMF under different ambient refractive index,temperature and incident wavelength are obtained.After that,a negative refractive index response（the interference spectrum blue shifts along with the increase of ambient refractive index）and a positive temperature response（the interference spectrum red shifts along with the increase of temperature）for the fundamental-cladding modal interference in the SMF are deduced.At the same time,a positive refractive index response（the interference spectrum red shifts along with the increase of refractive index）and a positive temperature response for the cladding-cladding modal interference in the SMF are analyzed.In addition,a temperature negative response of LP₀₁-LP₁₁interference in the FMF is also demonstrated.（2）The principle and method to obtaine an enhanced or reduced vernier effect using modal interference discrepancy are proposed.Sensitivity of the interferometer is determined by the response trend consistency of the two units.An enhanced sensitivity,that is,a enhanced vernier effect,could be obtained when the response trend of the two unit interferometers is inconsistent.A decreased sensitivity,that is,a decayed vernier effect,is related to an consistent response trend.The enhanced vernier effect is beneficial to the detection of the measured,while the decayed vernier effect is helpful to resrtain the crosstalk from the temperature fluctuation.（3）To excite specific modes and to obtain specific modal interference,excitation efficiencies of those main modes in SMF-SMF and SMF-FMF splicing with different splicing lateral-offset are calculated.For SMF-SMF splicing（8.2/125μm in core/cladding diameter）,a comparable excitation efficiency of fundamental mode and cladding modes,and accordingly a optimum contrast radio of fundamental-cladding modal interference occurs when the lateral-offset distance is 6μm.Excitation efficiency of fundamental mode decreases to zero and only cladding-cladding modal interference could be obtained when the lateral-offset distance in SMF-SMF splicing increases to 30μm.For the SMF-FMF splicing（8.2/125μm SMF and 14/125μm FMF in core/cladding diameter）,a comparable excitation efficiency of LP₀₁ and LP₁₁ mode,and accordingly a optimum contrast radio of LP₀₁-LP₁₁interference could be obtained when the lateral-offset distance of SMF-FMF splicing is 6μm.The exact modal interference amounts and type could be determined by frequency from the fast Fourier transformed interference spectrum.And a fiber micro-bending method could be adopted to adjust the free spectral range of a certain modal interference to the desired frequency.（4）Based on the blue shift ambient refractive index response of fundamental mode-cladding modal interference and the red shift refractive index response of cladding-cladding modal interference in SMF,a cascaded SMF-SMF-SMF in-line MZI with enhanced vernier effect for ambient refractive index sensing was designed,which contained two SMF-SMF-SMF units MZI with a slight（6μm）and an obvious（30μm）lateral-offset distance,respectively.Experiments verified that the 6μm lateral-offset unit MZI was dominated by fundamental mode-cladding modal interference,which presented a blue shift response of-54.009 nm/RIU to the increase of ambient refractive index（1.3362-1.3811）.The 30μm lateral-offset unit MZI,however,were mainly occupied by cladding-cladding modal interference and showed a red shift of 142.581nm/RIU to the increase of ambient refractive index.As for the cascaded interferometer,it presented an refractive index sensitivity as high as-1788.160 nm/RIU,which is 33.1and 12.5 times increased than that of the unit MZI with slight and obvious lateral-offset,respectively.Moreover,both the two units MZI had a red shift response to temperature increase,thus the cascaded interferometer turned into decayed vernier for temperature detection.The temperature sensitivity was measured to be 0.167 nm/℃,resulting in a small refractive index error of 9.339×10^-5 RIU/℃.（5）Based on the blue shift ambient temperature response of LP₀₁-LP₁₁interference in an FMF and the red shift temperature response of fundamental mode-cladding modal interference in a SMF,a cascaded interferometer with enhanced temperature sensitivity has been developed,which contains a SMF-FMF-SMF unit MZI and a SMF-SMF-SMF one,both of which have the same slight lateral-offset（6μm）.Temperature sensitivity of the cascaded vernier interferometer is-0.794 nm/℃,which is 11.5 and 14.4 times increased compared with that of a SMF-FMF-SMF unit MZI（-0.069 nm/℃）and a SMF-SMF-SMF MZI（0.055 nm/℃）,respectively.The results of this paper can provide theoretical and experimental reference for the research of In-Line MZI sensor with enhanced ambient refractive index or temperature sensitivity,decayed temperature cross talk,simple structure and convenient fabrication.